How Mars Works

Mars has fascinated us for millennia. Almost from the time astronomers first turned their telescopes on the planet shining in the night sky, we've imagined life there. Unlike our other planetary neighbor, Venus, which remains shrouded in cloudy mystery, the red planet has invited speculation and exploration. Since the 1960s, the U.S. and the Soviet Union and, later, Russia and Japan, have launched spacecraft destined to land on or orbit Mars.

The successful missions, like the very first Mars flyby in 1964 by the U.S. Mariner 4, have provided a treasure trove of data and, of course, introduced many new questions. Recently, those data, provided compliments of spacecraft such as the Phoenix Mars Lander, the Curiosity rover, and the Mars Reconnaissance Orbiter, have been arriving at Earth at a dizzying rate. It seems like a golden age for Mars exploration has arrived.

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Here's what we've learned about the fourth planet from the sun while orbiting it, landing on it and sampling its contents: It's cold, dusty and dry, but that probably wasn't always the case. Ample data seem to point toward liquid water rushing over its surface in the form of lakes, rivers and an ocean at some undetermined point in the past. Traces of methane have been detected in the atmosphere, but its source is unknown. On Earth, much of the methane is produced by living organisms, like cows, which could bode well for the possibility of life on Mars. On the other hand, the gas could also have nonbiological origins, such as the Martian volcanoes.

One thing we do know: Humans won't be walking on Mars anytime soon. All manner of robots will have cruised its dusty surface long before we do. The next best thing to exploring Mars is reading about it, right? So get ready to launch into the fascinating world of the red planet. How did it form? What's the weather like? And most important, has water or life ever existed on Mars?

Mars History

As you can see from the accompanying image, Mars has few distinguishing features when viewed from Earth, even with the best telescopes. There are dark and light areas, as well as polar ice caps, but certainly not the clear features that you can see in images from orbiters around Mars. Therefore, we can excuse early astronomers for making mistakes or embellishing their observations. To these scientists searching the sky, Mars was a vastly different world than we know today.

In 1877, Giovanni Schiaparelli, an Italian astronomer, became the first person to map Mars. His sketch showed a system of streaks or channels, which he called canali. In 1910, the U.S. astronomer Percival Lowell made observations of Mars and wrote a book. In his book, Lowell described Mars as a dying planet where the civilizations built an extensive network of canals to distribute water from the polar regions to bands of cultivated vegetation along their banks.

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Although Lowell's book captured the public's imagination, the scientific community summarily dismissed it because his observations weren't confirmed. Nevertheless, Lowell's writings sparked generations of science fiction writers. Edgar Rice Burroughs of Tarzan fame wrote several novels about Martian societies, including "The Princess of Mars," "The Gods of Mars" and "The Warlord of Mars." H.G. Wells wrote "The War of the Worlds" about invaders from Mars (Orson Welles' radio play of this book caused a national panic in 1938).

Hollywood has also fueled the public's fascination with the planet in films such as "The Angry Red Planet," "Invaders from Mars" and, more recently, "Mission to Mars," two versions of "Total Recall," and a live-action version of Burroughs' titular hero in "John Carter."

In the 1960s and 1970s, however, the American Mariner, Mars and Viking missions started sending back images of a world very different from that described by Lowell and his literary and silver-screen successors. The photos, snapped during flybys of the planet and eventually during the Viking landings, showed Mars as a dry, barren, lifeless world with variable weather that often included massive dust storms that could whip across a majority of the planet. So with thousands of photos as evidence, Mars was confirmed as a desert planet with rocks and boulders, rather than the home of irritable Martians and man-eating plants a la "The Angry Red Planet."

Now, we have extensively mapped the planet with Mars Global Surveyor, sent rovers to bump over its surface and scoop up soil samples, and launched orbiters to observe the planet from space. More missions are in the works. NASA and the European Space Agency (ESA) have committed to continued robotic and possibly human exploration of Mars.

So far these missions have enabled scientists to hazard a theory about how the red planet formed, and the story would actually make a pretty good movie. Read on to learn how solar system collisions gave Earth its next-door neighbor.

The Origins of Mars

Unfortunately, no human geologist has been to Mars. So the best information that we have about the planet's beginnings 4.6 billion years ago comes from images taken by orbiters and landers, Martian meteorites, and comparisons with its planetary peers (Mercury, Venus, Earth and Earth's moon). The current theory goes like this:

Mars formed from clumping or accretion of small objects in the early solar system.

However, unlike Earth and Venus, Mars finished forming within 2-4 million years and never grew beyond the planetary embryo stage.

Possibly, aluminum 26 decay turned the planet into a magma ocean.

After cooling, there was a period of intense bombardment from meteors.

The hot mantle pushed through and lifted portions of the crust.

One or more periods of intense volcanic activity and lava flows followed.

The planet cooled and the atmosphere thinned.

Let's look at these steps in more detail.

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Mars was created by the accretion of small objects in the early solar system, which took about 2-4 million years. Mars grew and developed a larger gravity field, which attracted more bodies. These bodies would fall into Mars, impact and generate heat. Some models suggest that such heating would not have been enough to bring about large-scale melting on Mars; rather, because the planet formed so quickly, it might have gobbled up enough of the aluminum 26 nuclide, which has a half-life of only 717,000 years, to melt from radioactive decay. Gradually, the material sorted itself out into a core, mantle and crust. Gases released from the cooling formed a primitive atmosphere [source: Dauphas and Pourmand].

But as an embryo planet formed in the solar system's chaotic early days, Mars couldn't catch a break. It was heavily bombarded by meteors in the inner solar system. These bombardments produced craters and multi-ring basins all over the planet, like the 1,400-mile- (2,300-kilometer-) wide Hellas Planitia impact crater in the planet's southern hemisphere. Some geologists think that a huge impact occurred that thinned the crust of the northern hemisphere. Similar impacts occurred on Earth and our moon at this same time. On Earth, the craters were eroded by wind and water. On the moon, the evidence of these great collisions is still visible.

Now imagine Mars is a soft-boiled egg; the inside is hot as the shell cools. If the shell is weak in spots, the egg will crack and the cooked yolk will protrude. A similar occurrence happened with the Tharsis region, a continent-sized land mass in the southern hemisphere. The hot mantle bulged out, pushing up the crust and fracturing the surrounding lava plains (forming Valles Marineris, a network of canyons). In other spots, the mantle pushed through the crust, giving rise to the region's many volcanoes, such as Olympus Mons. (We'll talk about all these Martian landmarks next.)

During this period, there were widespread volcanic eruptions. Lava flowed from volcanoes and filled the low-lying basins. Eruptions released gas that contributed to a thick atmosphere, which could have supported liquid water. Therefore, there might have been rain, flooding and erosion. The erosion would produce sedimentary rocks in the basins and plains, and form channels in the rock. More than one period of widespread volcanic eruptions may have occurred during Mars' history, but eventually the volcanoes stopped rumbling as much.

The bulges that caused the crustal uplifts and the widespread volcanic activity released vast amounts of heat from the inside of Mars. Since Mars isn't as large as the Earth, it cooled much faster, and the surface temperature cooled with it. Water and carbon dioxide from the atmosphere began to freeze and fall to the surface in vast quantities. This freezing removed large amounts of gas from the atmosphere, causing it to thin. In addition, any surface water may have frozen into the ground, forming permafrost layers. Intermittent volcanic eruptions would release more heat that would melt more water ice and cause flooding. The flooding would erode channels and carry more material down to the surrounding plains.

As for the rest of Mars' atmosphere, it was likely blown off under the assault of solar wind. Earth's magnetic field protects us from the worst of such effects, but the Mars equivalent shut down around 4 billion years ago, possibly due to a series of massive asteroid impacts that threw off the temperature gradient powering the planetary electric dynamo [source: Than].

While this is the current theory about the origin of Mars, it needs more data to back it up.

The southern highlands are extensive. The region's elevated terrain is heavily cratered like the moon. Scientists think the southern highlands are ancient because of the large number of craters. Most cratering in the solar system happened more than 3.9 billion years ago, at which point the rate of meteors bashing into the solar system's planetary bodies dropped steeply.

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The northern plains are low-lying regions, much like the maria, or seas, on the moon. The plains show lava flows with small cinder cones -- evidence of volcanoes -- as well as dunes, wind streaks, and major channels and basins similar to dry "river valleys." There is a distinct change in elevation, of several kilometers, between the southern highlands and the northern plains.

Two continent-sized, high regions called crustal upwarps spread over the northern plains. In these upwarps areas the molten rock from the interior mantle pushed up the planet's thin crust, forming a high plateau. These regions are capped with shield volcanoes, where molten rock from the magma broke through the crust. The smaller region, named Elysium, is in the eastern hemisphere, while the larger one, called Tharsis, is located in the western hemisphere.

The highest point in the solar system that we know about rises up in the Tharsis region. This shield volcano called Olympus Mons (Mount Olympus from Greek mythology) towers 16 miles (25 kilometers) above the surrounding plains, and its base spans 370 miles (600 kilometers). In contrast, the largest volcano on Earth is Mauna Loa in Hawaii, which rises 6 miles (10 kilometers) above the ocean floor and is 140 miles (225 kilometers) wide at its base.

At the edge of the Tharsis region is a large system of canyons called Valles Marineris. The canyons stretch for 2,500 miles (4,000 kilometers). That's greater than the distance from New York to Los Angeles. The canyons are 370 miles (600 kilometers) wide and 5 to 6 miles (8 to 10 kilometers) deep. That makes Valles Marineris much larger than the Grand Canyon. Unlike the U.S. national landmark, which formed from water erosion from the Colorado River, Valles Marineris was created by the crust cracking when the Tharsis bulge formed.

We can see the polar regions from the Earth. Surrounded by vast dunes, the northern and southern polar ice caps seem to be made mostly of frozen carbon dioxide (dry ice) with some water ice. Like Earth, Mars has an axial tilt that causes it to experience seasons. The size of the polar ice caps varies with the season. In the summer, the carbon dioxide from the northern ice cap sublimes, or turns directly from ice to steam, revealing a sheet of water ice below. In fact, the water ice in this northern region is the reason why NASA sent the Phoenix lander there. With the help of its robotic arm, Phoenix dug down to the frozen layer and examined soil samples to investigate its composition.

The Interior of Mars

Let's compare the interior of Earth with that of Mars. Earth has a core with a radius of about 2,200 miles (3,500 kilometers) -- roughly the size of the entire planet of Mars. It is made of iron and has two parts: a solid inner core and a liquid outer core. Radioactive decay in the core generates the heat. This heat is lost from the core to the layers above. Convective currents in the liquid outer core along with the rotation of Earth produce its magnetic field.

Mars, the more petite planet, probably has a core radius between 900 and 1,200 miles (1,500 kilometers and 2,000 kilometers). Its core is probably made of a mixture of iron, sulfur and maybe oxygen. The outer part of the core may be molten, but it's unlikely, because Mars has only a weak magnetic field (less than 0.01 percent of Earth's magnetic field). Although Mars doesn't have a strong magnetic field now, it might have had a powerful one long ago.

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Surrounding Earth's core is a thick layer of soft rock called the mantle. What do we mean by soft? Well, if the outer core is liquid, then the mantle is a paste, like toothpaste. The mantle is less dense than the core (which explains why it rests above the core). It's made of iron and magnesium silicates, and it stretches about 1,800 miles (3,000 kilometers) thick -- remember that the next time you try to dig a hole to China). The mantle is the source of lava that spews and trickles from volcanoes.

Like Earth, the mantle of Mars (the wide grayish-brown swath in the figure) is probably made of thick silicates; however, it's much smaller, at 800 to 1,100 miles (1,300 to 1,800 kilometers) thick. There must have been convective currents that rose up in the mantle at one time. These currents would account for the formation of the crustal upwarps, such as the Tharsis region, the Martian volcanoes and the fractures that formed Valles Marineris.

On Earth, the crust's continental plates float over the underlying mantle and rub against each other (continental drift). The areas where they rub produce uplift, cracks or faults, such as the San Andreas fault in California. These areas of contact between plates experience earthquakes and volcanoes. On Mars, the crust is also thin, but isn't broken into plates like the Earth's crust. Although we do not know of currently active volcanoes or marsquakes, evidence of quakes occurring as recently as a few million years ago suggest they are possible [source: Spotts].

Do you want to see all this for yourself? You might have difficulty breathing on Mars. Find out why next.

The Atmosphere of Mars

Of all the planets, Mars is our closest relation in terms of makeup (not distance -- Venus is closer), but that's not saying much. And it certainly doesn't mean that it is hospitable. The atmosphere of Mars differs from Earth's in many ways, and most of them don't bode well for humans living there.

It's composed mostly of carbon dioxide (95.3 percent compared to less than 1 percent on Earth).

Mars has much less nitrogen (2.7 percent compared to 78 percent on Earth).

It has very little oxygen (0.13 percent compared to 21 percent on Earth).

The red planet's atmosphere is only 0.03 percent water vapor, compared to Earth, where it makes up around 1 percent.

On average, it exerts only 6.1 millibars of surface pressure (Earth's average sea-level atmospheric pressure is 1,013.25 millibars) [source: NASA].

Because the "air" on Mars is so thin, it holds little of the heat that comes from the ground after it absorbs solar radiation. The thin air also is responsible for the wide, daily swings in temperature (almost 100 degrees Fahrenheit or 60 degrees Celsius). Martian atmospheric pressure changes with the seasons. During the Martian summer, carbon dioxide sublimes from the polar ice caps into the atmosphere, increasing the pressure by about 2 millibars. As found by NASA's Mars Reconnaissance Orbiter, during the Martian winter, carbon dioxide refreezes and falls from the atmosphere as carbon dioxide snow! This snowfall causes the pressure to decrease again. Finally, because the Martian atmospheric pressure is so low and the average temperature is so cold, liquid water cannot exist; under these conditions, water would either freeze, evaporate into the atmosphere or, as seen by NASA's 2008 Phoenix Lander mission, fall as snow [source: NASA].

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The weather on Mars is pretty much the same each day: cold and dry with small daily and seasonal changes in temperature and pressure, plus a chance of dust storms and dust devils [source: NASA]. Light winds blow from one direction in the morning and then from the reverse direction in the evening. Clouds of water ice hover at altitudes of 12 to 18 miles (20 to 30 kilometers), and clouds of carbon dioxide form at approximately 30 miles (50 kilometers). Because Mars is so dry and cold, it never rains. That's why Mars resembles a desert, much like Antarctica on Earth.

During the spring and early summer, the sun heats up the atmosphere enough to cause small convection currents. These currents lift dust into the air. The dust absorbs more sunlight and heats the atmosphere further, causing more dust to lift into the air. As this cycle continues, a dust storm develops. Because the atmosphere is so thin, great speeds (60 to 120 mph or 100 to 200 kph) are required to stir up the dust. These dust storms spread across large regions of the planet and can last for months. All that dust can be bad for the rovers traversing the surface, but the storms can also clear off dirt caked on their solar panels.

Dust storms are also thought to be responsible for the variable dark regions on Mars that are seen from ground-based telescopes, which were mistaken for canals and vegetation by Percival Lowell and others. The storms are also a major source of erosion on the Martian surface.

Is all that dust making you thirsty? Read on to find out about water on Mars.

Water on Mars

Liquid water is essential for life, at least here on Earth. Presumably, the same goes for arid Mars. Or that's the assumption that governed NASA's "follow the water" strategy for Mars exploration.

Scientists don't think the liquid was always so scarce. Modern Mars may resemble a barren desert, but very early Mars may have been quite wet, judging from some of the geologic clues left behind. Floods may once have flowed over the planet's surface, rivers may have carved out channels or gullies, and lakes and oceans may have covered large swaths of the planet.

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Evidence for this has vastly increased in recent years, with the observations of the Mars Reconnaissance Orbiter, which found thousands of deposits of phyllosilicates at locations around the planet. These claylike minerals arise solely in watery environments -- at temperatures friendly to life -- but were probably laid down in the early days of the solar system, around 4.6 to 3.8 billion years ago. Rovers like Opportunity and Curiosity have revealed that at least some of these lakes maintained salt and acidity levels friendly to life [sources: Rosen; Yeager].

Can't quite picture it? Visit Mono Lake in California, one of the world's oldest lakes at 760,000 years old and an average of 57 feet (17 meters) deep. Now imagine it without water and you'll have the Gusev Crater, a giant basin bisected by a dry riverbed that the Spirit rover searched for evidence of water.

When scientists looked at high-resolution, 3-D images of Mars taken in 2005 and compared them to pictures taken in 1999 of the same area, what they saw excited them: A series of bright, depositary streaks had formed in gullies during the intervening years. These streaks were reminiscent of flash floods that can carve away soil and leave behind new sediments on Earth. A bunch of streaks doesn't sound that monumental, but if water was the recent force behind them, that changes things. (To learn more about the discovery, read "Is there really water on Mars?")

Liquid water may be in short supply, but frozen water isn't. The Phoenix lander investigated the ice in the far north of Mars. The lander's robotic arm dug down into the icy layer for soil samples, which it analyzed with its onboard instruments.

In fact, the lander had three main objectives, all of them water-related:

Study the history of water in all its phases.

Determine if the Martian arctic soil could support life.

Study Martian weather from a polar perspective.

Life on Mars?

This green guy might be what you're picturing when you think about life on Mars, but microbes are the more realistic possibility.

Antonio M. Rosario/Getty Images

This simple question has captivated minds for centuries. We still lack a definitive answer, although evidence has continued to mount as spacecraft carry out increasingly sophisticated tests for life processes, past and present, including analyzing Martian soil for traces of water and looking for the release of gases such as carbon dioxide, methane and oxygen that might suggest bacterial life.

It's possible that we need to revisit our idea of Martian life, exchanging egg-headed humanoids for much smaller organisms. Microbes are hardy little buggers, and there's good reason to believe that they could exist below ground. For example, biologists have unearthed bacteria living in Antarctica as well as a species, dormant for 120,000 years and buried 2 miles (3.2 kilometers) below Greenland's ice, that successfully awoke from its frozen slumber and started multiplying [source: Heinrichs].

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There's also plenty of evidence that Mars' environment billions of years ago could have supported them. As we discussed, water is a key ingredient for life, and we know that Mars used to be wet. Curiosity rover was dispatched to Gale Crater because it marks a spot where water flowed for a long period. This history is recorded in the layer after layer of sediment that built its central feature, the 3.4-mile- (5.5-kilometer-) high Mount Sharp (aka Aeolis Mons), over billions of years [sources: Drake; Yeager].

Indeed, 10 years into its mission, Opportunity found another spot like Gale Crater where ancient water was not too acidic or salty for cells to flourish. Moreover, although Curiosity's drill has yet to locate the organic carbon compounds that would form life-related amino acids, it has dug up hydrogen, carbon, sulfur, nitrogen, phosphorous and oxygen -- a well-stocked pantry for single-celled organisms, if they did exist. Back on Earth, scientists have found Mars meteorites with internal structures that are consistent with a biological source [sources: Grant; NASA; Rosen].

In short, there's plenty of evidence that Mars was friendly to life long ago, but no smoking gun. Even if there were, we have to ask: Could it still be hanging around somewhere?

One promising sign of life would be the discovery of large amounts of methane in the Martian atmosphere. Scientists had previously detected the gas -- 90-95 percent of which on Earth is produced by microbes -- in Mars' atmosphere. They hypothesized that trapped methane from buried microorganisms might be released during seasonal ground thaws. So far, Curiosity's measurements indicate levels 1/10,000 of those found in Earth's atmosphere -- in other words, bupkes -- but, given more time, there's a slight chance that the rover might observe such a seasonal bloom. Then again, the methane clouds observed by scientists could arise from a natural process, such as the release of methane trapped in ice [sources: Savage; Wayman].